Selective laser melting (SLM) and selective laser sintering (SLS) apparatus produce objects through layer-by-layer solidification of a material, such as a metal powder material, using a high energy beam, such as a laser beam. A powder layer is formed across a powder bed in a build chamber by depositing a heap of powder adjacent to the powder bed and spreading the heap of powder with a wiper across (from one side to another side of) the powder bed to form the layer. A laser beam is then scanned across areas of the powder layer that correspond to a cross-section of the object being constructed. The laser beam melts or sinters the powder to form a solidified layer. After selective solidification of a layer, the powder bed is lowered by a thickness of the newly solidified layer and a further layer of powder is spread over the surface and solidified, as required. An example of such a device is disclosed in U.S. Pat. No. 6,042,774.
Typically, the laser beam is scanned across the powder along a scan path. An arrangement of the scan paths will be defined by a scan strategy. U.S. Pat. No. 5,155,324 describes a scan strategy comprising scanning an outline (border) of a part cross-section followed by scanning an interior (core) of the part cross-section. Scanning a border of the part may improve the resolution, definition and smoothing of surfaces of the part.
It is known to use a continuous mode of laser operation, in which the laser is maintained on whilst the mirrors move to direct the laser spot along the scan path, or a pulsed mode of laser operation, in which the laser is pulsed on and off as the mirrors direct the laser spot to different locations along the scan path.
The strategy used for scanning a part can affect the thermal loads generated during the build and accuracy of the resultant solidified line of material.
Excessive, unrestrained thermal stresses created during the build causes warping and/or curling of the part being built. As solidified material cools, the temperature gradient across the cooling solidified material can cause warping and/or curling of the part. U.S. Pat. No. 5,155,324 and US2008/0241392 A1 describe scanning an area in a plurality of parallel scan paths (raster scan). The direction of the scan paths are rotated between layers to homogenise tensions generated during the build. US2008/0241392 A1 extends this concept to scanning in a series of parallel stripes, wherein each stripe consists of a plurality of parallel scan paths running perpendicular to a longitudinal direction of the stripe. The direction of the stripes are rotated by 67 degrees between layers.
US2005/0142024 discloses a scan strategy for reducing thermal loads comprising successively irradiating individual areas of a layer, which are at a distance from one another that is greater than or at least equal to a mean diameter of the individual areas. Each individual area is irradiated in a series of parallel scan paths.
A melt pool generated by the laser is dependent upon the properties of the material and the state (powder or solidified) and temperature of material surrounding the volume being melted. The scan strategy used can affect the state and temperature of the neighbouring material. For example, scanning of the laser spot along a scan path in continuous mode forms a large melt pool that is dragged along just behind the laser spot, resulting in larger, less detailed solidification lines. For some materials, such as tool steels and aircraft grade super alloys, it can be difficult to drag the melt pool across the layer in a continuous mode of operation of the laser. These problems can be mitigated by using the laser beam in the pulsed mode of operation. In particular, setting the time between pulses to be long enough to allow a previously formed melt pool to cool before forming an adjacent melt pool can result in more accurate solidification lines, which may be particularly beneficial for border scans. However, slowing the scans to this extent can significantly increase the time to scan that area/path and therefore, significantly increase the build time.